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HA means: group, faction, sect, school of thought; to send or despatch someone. For example, a haken shihan is a shihan who has been sent somewhere to teach aikido. A habatsu is a faction or clique.

The character often occurs as the second or third of a compound word, as in 'ryuha' (school of thought or martial arts), 'gakuha' (school, in a more academic sense), 'hatoha' ("doves"), 'takaha' ("hawks"), 'rittaiha' (cubists), 'inshoha' (impressionist school), 'kotoha' (transcendentalists), and 'kagekiha' (radicals).

To build on what Peter said, "ha" seems to carry more the connotation of a group of individuals who share something in common, while "ryuu" carries more the connotation of the style (of aikido, painting, etc) that they practice.

As individual characters, either typically works as a suffix, not really as a stand-alone nominal (noun). If you want a noun that stands alone, it is most natural (normal) to use the compound "ryuuha".